1. Field of the Invention
The present invention relates to an improvement in a dichroic mirror device for separating/synthesizing a red light (R), a green light (G) and a blue light (B), which device is used for a projector such as a liquid crystal projector. The present invention also relates to a projection display apparatus using such a dichroic mirror device.
2. Description of the Related Art
Liquid crystal projectors, CRT (Cathode Ray Tube) projectors and CCD (Charged Coupled Device) cameras are accommodated in rear projection televisions, front projection televisions or television cameras. Projectors employing liquid crystal panels or CRTs for the above colored lights R, G or B, require a mechanism to separate or synthesize lights. There is a device for light separation/synthesis using a glass prism. However, the glass prism is actually used only for 1 to 2 inch liquid crystal panels. If employing a glass prism for a larger liquid crystal panel or a CRT, the glass prism has to be very large in size with an increased cost. Furthermore, there may arise the problem of retardation (irregular phase difference), when a large glass prism is used.
There is another device for light separation/synthesis, which uses three dichroic mirrors instead of a glass prism. FIG. 14 shows such a cross-type three color separation/synthesis mirror device 4. The device comprises a dichroic mirror 1 and two otherdichroic mirrors 2 and 3 holding the mirror 1 therebetween, which forms an X-shape. A blue light B is emitted from a CRT 5, a green light G from a CRT 6 and a red light R from a CRT 7, respectively. Those three colored lights are synthesized by the separation/synthesis mirror device 4, pass through a lens 8 and are projected to the screen (not shown in FIG. 14) to form an image thereon.
However, a light passing through the joint portion 9 goes along a different path from that of a light passing through other portions of the device, which adversely affects the image quality. For example, a double image appears around the center of the screen, the image gets blurred or it lacks clearness, resulting in decrease in number of scanning lines.
The other problem is an out-of-focus image and a decline in image contrast caused by an increase in the astigmatism. The astigmatism increases because the dichroic mirrors 1 to 3 are placed obliquely to the CRTs 5 to 7.